xwayland/glamor: Track if a xwl_pixmap uses explicit modifiers

If we allocated with implicit modifiers, then we shouldn't use the modifier returned by gbm_bo when checking whether the modifier is supported or not, since it won't be if the compositor only advertises implicit modifiers, nor should we use the modifier when creating the Wayland buffer object, as it wasn't explicitly advertised.

Fixes: c6f2598a ("xwayland: don't fall back to wl_drm with explicit modifier")